Citing documents provided by Snowden, it claimed the NSA and its U.K. equivalent hacked into Gemalto to get hold of the encryption keys that allow access to SIM cards across the globe.

Based in the Netherlands, Gemalto reportedly makes two billion SIM cards per year. Clients include AT&T(T), T-Mobile(TMUS), Verizon(VZ), Sprint(S) and about 450 other global telecom firms, according to the report. It also makes chips for credit cards and works with over 3,000 financial institutions.

The company, which prides itself on its top-notch digital security, said it was not aware of the hacking, which ran from 2010 to 2011. It is investigating the report.

"We cannot at this early stage verify the findings of the publication and had no prior knowledge that these agencies were conducting this operation," the firm said in a statement.